The present invention relates to an automatic noise removal device for removal of high frequency components from a picture signal only when the S/N ratio is bad in a television receiver.
A noise component contained in a television signal generally exists in high frequencies of the television signal. Therefore, a mere removal of the noise may be attained by attenuating the high frequencies of the television signal.
The attenuation of the high frequencies, however, brings about a loss of the information representing the contour of the picture being reproduced. Therefore, the resolution of the reproduced picture is deteriorated, resulting in a significant reduction in the picture quality. For this reason, it is required that the high frequencies with higher amplitudes than that of the noise be left in the picture signal to the extent possible.
A technique to satisfy this requirement is disclosed in U.S. Pat. No. 4,031,547. The prior art technique adjusts the amount of attenuation of the high frequencies in accordance with the level of the picture signal, thereby to successfully remove only the noise with relatively small deterioration of the resolution. In a picture reproduced by a television receiver, a deterioration of the resolution in a dark portion of the reproduced picture contributes little to a deterioration of the picture quality, but this portion of the picture is greatly influenced by noise. Based on this fact, when the picture is dark, that is, when the level of the picture signal is lower as seen from a black level, the high frequencies are attenuated relatively greatly. When the picture becomes bright, that is, when the level of the picture signal becomes high, the high frequencies are transmitted and not attenuated. In this way, only the noise component may be effectively removed while deteriorating the picture quality only to a relatively small degree.
According to this method, however, the high frequency component is always lost in the dark portion. Therefore, even in a noise-free condition with a good S/N ratio, the resolution must always be reduced in some degree.